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<html>
  <head>
    <meta charset='utf-8'>
    <meta name='viewport' content='width=device-width, initial-scale=1, user-scalable=no'>
    <meta name='mobile-web-app-capable' content='yes'>
    <meta name='apple-mobile-web-app-capable' content='yes'>

    <!-- Origin Trial Token, feature = WebXR Device API (For Chrome M76+), origin = storage.googleapis.com, expires = 2019-07-24 -->
    <meta http-equiv="origin-trial" data-feature="WebXR Device API (For Chrome M76+)" data-expires="2019-07-24" content="Ap6io/uhkGK7vXCD+golNnQfj8wJ4so790EzZoqb8YOljMXIBTvBEQFPTHYIz5d/BgtuwZTKOLrmHAOt30f38g8AAABxeyJvcmlnaW4iOiJodHRwczovL3N0b3JhZ2UuZ29vZ2xlYXBpcy5jb206NDQzIiwiZmVhdHVyZSI6IldlYlhSRGV2aWNlTTc2IiwiZXhwaXJ5IjoxNTY0MDA5MzU2LCJpc1N1YmRvbWFpbiI6dHJ1ZX0=">

    <title>Barebones WebXR</title>
    <link href='css/common.css' rel='stylesheet'></link>
  </head>
  <body>
    <header>
      <details open>
        <summary>Barebones WebXR</summary>
        <p>
          This sample demonstrates extremely simple use of WebXR with no library
          dependencies. It doesn't render anything exciting, just clears your
          headset's display to a slowly changing color to prove it's working.
          <a class="back" href="./index.html">Back</a>
        </p>
        <div id="warning-zone"></div>
        <button id="xr-button" class="barebones-button" disabled>XR not found</button>
      </details>
    </header>
    <main style='text-align: center;'>
      <p>Click 'Enter XR' to see content</p>
    </main>
    <script type="module">
      // XR globals.
      let xrButton = document.getElementById('xr-button');
      let xrSession = null;
      let xrRefSpace = null;

      // WebGL scene globals.
      let gl = null;

      function checkSupportedState() {
        // If the device allows creation of immersive VR sessions, enable the
        // button. If the supportsSession request is rejected, then
        // disable the button because it means that the desired session mode is
        // not supported.
        navigator.xr.isSessionSupported('immersive-vr').then((supported) => {
          // Updates the button to start an XR session when clicked.

          if (supported) {
            xrButton.innerHTML = 'Enter XR';
          } else {
            xrButton.innerHTML = 'XR not found';
          }

          xrButton.disabled = !supported;
        });
      }

      // Checks to see if WebXR is available. If it is, checks if the desired
      // session options are supported both right now and whenever a device is
      // added or removed.
      function initXR() {
        // WebXR API is only available for secure contexts. If the page is using
        // an insecure context, add a warning message about this above the
        // button.
        if (!window.isSecureContext) {
          let message = "WebXR unavailable due to insecure context";
          document.getElementById("warning-zone").innerText = message;
        }

        // If WebXR is available on this UA, check if any devices are connected
        // that would support an immersive VR session.
        if (navigator.xr) {
          // Register our click handler.
          // In the other WebXR samples, most of the button details are handled
          // by XRDeviceButton from js/webxr-button.js.
          xrButton.addEventListener('click', onButtonClicked);

          // Register for device change events. This indicates that a device has
          // been added or removed, which means that whether or not XR is
          // supported has likely changed.
          navigator.xr.addEventListener('devicechange', checkSupportedState);

          // Just in case the devicechange event has already fired, call it now
          // as well.
          checkSupportedState();
        }
      }

      // Called when the user clicks the button to enter XR. If we don't have a
      // session already we'll request one, and if we do we'll end it.
      function onButtonClicked() {
        if (!xrSession) {
          navigator.xr.requestSession('immersive-vr').then(
            onSessionStarted,
            onRequestSessionError);
        } else {
          xrSession.end();
        }
      }

      // Called when we've successfully acquired a XRSession. In response we
      // will set up the necessary session state and kick off the frame loop.
      function onSessionStarted(session) {
        xrSession = session;
        xrButton.innerHTML = 'Exit XR';

        // Listen for the sessions 'end' event so we can respond if the user
        // or UA ends the session for any reason.
        session.addEventListener('end', onSessionEnded);

        // Create a WebGL context to render with, initialized to be compatible
        // with the XRDisplay we're presenting to.
        let canvas = document.createElement('canvas');
        gl = canvas.getContext('webgl', {
          xrCompatible: true
        });

        // Use the new WebGL context to create a XRWebGLLayer and set it as the
        // sessions baseLayer. This allows any content rendered to the layer to
        // be displayed on the XR Device.
        session.updateRenderState({ baseLayer: new XRWebGLLayer(session, gl) });

        // Get a reference space, which is required for querying poses. In
        // this case a 'local' reference space means that all poses will be
        // relative to the location of the user's head at the time this
        // reference space is first created.
        session.requestReferenceSpace('local').then((refSpace) => {
          xrRefSpace = refSpace;

          // Inform the session that we're ready to begin drawing.
          session.requestAnimationFrame(onXRFrame);
        });
      }

      // Called when the requestSession promise was rejected with an exception.
      function onRequestSessionError(ex) {
        alert("Failed to start immersive VR session.");
        console.error(ex.message);
      }

      // Called when the user clicks the 'Exit XR' button. In response we end
      // the session.
      function onEndSession(session) {
        session.end();
      }

      // Called either when the user has explicitly ended the session (like in
      // onEndSession()) or when the UA has ended the session for any reason.
      // At this point the session object is no longer usable and should be
      // discarded.
      function onSessionEnded(event) {
        xrSession = null;
        xrButton.innerHTML = 'Enter VR';

        // In this simple case discard the WebGL context too, since we're not
        // rendering anything else to the screen with it.
        gl = null;
      }

      // Called every time the XRSession requests that a new frame be drawn.
      function onXRFrame(t, frame) {
        let session = frame.session;

        // There are a few advantages to requesting the next frame immediately
        // instead of at the end of this function.
        // 1) It's less likely to forget to call requestAnimationFrame in cases
        //    when onXRFrame has more complicated control flow.
        // 2) If an exception is thrown later in this function, the render loop
        //    will still continue.
        session.requestAnimationFrame(onXRFrame);

        // Get the viewer pose relative to the reference space we created
        // earlier.
        let pose = frame.getViewerPose(xrRefSpace);

        // Getting the pose may fail if, for example, tracking is lost. So we
        // have to check to make sure that we got a valid pose before attempting
        // to render with it. If not, in this case we'll just leave the
        // framebuffer cleared, so tracking loss means the scene will simply
        // disappear.
        if (pose) {
          // If we do have a valid pose, bind the WebGL layer's framebuffer,
          // which is where any content to be displayed on the XR Device must be
          // rendered.
          gl.bindFramebuffer(gl.FRAMEBUFFER, session.renderState.baseLayer.framebuffer);

          // Update the clear color so that we can observe the color in the
          // headset changing over time.
          let time = Date.now();
          gl.clearColor(Math.cos(time / 2000), Math.cos(time / 4000), Math.cos(time / 6000), 1.0);

          // Clear the framebuffer
          gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

          // Normally you'd loop through each of the views reported by the frame
          // and draw them into the corresponding viewport here, but we're
          // keeping this sample slim so we're not bothering to draw any
          // geometry.
          /*for (let view of pose.views) {
            let viewport = session.renderState.baseLayer.getViewport(view);
            gl.viewport(viewport.x, viewport.y,
                        viewport.width, viewport.height);
            // Draw something.
          }*/
        }
      }

      // Start the XR application.
      initXR();
    </script>
  </body>
</html>
